A computational model is a framework for doing computations according to certain specified rules on some input data. These models come for example from automata theory, formal language theory, logic, or circuit theory. The computational power of such a model can be judged by evaluating certain problems with respect to that model. The theory of computations is the study of the inherent difficulty of computational problems, that is, their computational complexity. This monograph analyzes the computational complexity of the satisfiability, equivalence, and almost-equivalence problems with respect to various computational models. In particular, Boolean formulas, circuits, and various kinds of branching programs are considered.